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1. Field of the Invention
This invention relates to the art of weighing apparatus, and more specifically to weighing apparatus that has designs and methods for load cell failure prediction.
2. Description of Prior Art
Analog load cell systems can fail catastrophically due to lightning, water ingress, or mechanical shock, but more often the failure is by degrees over a period of time. The current method for the prediction of load cell function and prediction of failure involves the need of a service personnel to either visit the site of the weighing apparatus or to ship the weighing apparatus to a service center. This system is expensive, labor-intensive and has the potential for the introduction of errors based on human errors. It may require the weighing apparatus to be unavailable for an unacceptable period of time.
Many weighing applications require the use of multiple load cells in a single scale or in a number of associated scales. For example, a heavy capacity scale for weighing truck or railroad cars requires multiple load cells. Each load cell provides an analog signal proportional to the portion of the load borne by that load cell. Strain gages connected in a wheatstone bridge configuration often provide the analog signal. In heavy capacity applications the load is usually distributed over at least four load cells and some applications may require sixteen or more load cells. The sum of the load cell output signals must be obtained to provide a signal representative of the total load. The usual technique for summing the signals from the analog cells has been to connect the outputs in parallel to provide a single analog output signal representative of the total weight applied to the scale.
The weighing accuracy of multiple load cell scales depends not only on the accuracy of the individual cells but also on the mechanical and electrical interaction among them. Since the load cells usually have different sensitivities to applied loads the total scale output is usually dependent upon the position of the weight on the scale.
U.S. Pat. No. 4,261,195, to Lockery, U.S. Pat. No. 4,574,899 to Griffen and U.S. Pat. No. 4,556,115 to Lockery address the problem of load position compensation in multi-load cell scales.
When the analog circuits of multiple load cells are connected together they are essentially impossible to monitor individually. Thus, xe2x80x9ctrouble shootingxe2x80x9d or xe2x80x9cload cell failure predictionxe2x80x9d or repair of a scale can require disassembly of the electrical circuits in order to test the load cells individually and find the defective one. Further, when a load cell is replaced for any reason the scale often requires recompensation for load position. A known test weight is required to accomplish this recompensation. For large scales in particular this is a time consuming procedure and the known weight often inconvenient to obtain.
Recently there has appeared the so-called xe2x80x9cdigital load cellxe2x80x9d in which an analog-to-digital converter and microprocessor are dedicated to a single load cell. The electronic circuits are mounted on a printed circuit board connected directly to the load responsive spring element, or counterforce, of the load cell. Temperature, creep and linearity errors of the individual load cell have been compensated by digital techniques. There still is a need to better predict load cell failure.
The failure of a load cell in a weighing apparatus can cause a company to lose money and goodwill due to downtime and possible inaccurate readings, being able to predict this failure and to plan around it would be extremely valuable. ISO 9000 adds the need to store critical data such as weighing instrument calibration and to be able to track performance data.
The need for scales that automate compliance testing to ISO 9000 requirements while having better and more efficient load cell diagnostics and failure predictions shows that there is still room for improvement in the art.
1. Field of the Invention
U.S. Class 177-25.11
2. Description of related art including information disclosed under 37 CFR xc2xa7 1.97** greater than  and 1.98 less than .
An object of the present invention is to provide a better and more efficient method to predict potential load cell failures and meet ISO 9000 requirements for weighing apparatus.
The weighing apparatus and process, according to one aspect of the present invention, includes a new junction box that contains multiple A/D PCBs (each handling one analog load cell), a new Calibration management utility to meet ISO 9000 requirements, and has the xe2x80x9cload cell symmetryxe2x80x9d, xe2x80x9cempty platformxe2x80x9d and xe2x80x9ctest loadxe2x80x9d readings for each load cell stored on the weighing apparatus. This weighing apparatus makes periodic tests to determine the health of the load cells in the system, producing a log file to record events such as any detected failure, suspicious reading, overload condition, or any user defined parameter, and is network enabled to be connected through a LAN via an Ethernet to the Internet.
It is a general object of the present invention to substantially eliminate the problems described above associated with prediction of load cell failures in multiple load cell weighing apparatus. A further object is to provide compensation for load position and other errors in multiple load cell scales in which the analog portions of the load cells remain isolated.
The current invention has the use of an A/D converter for each load cell in a multiple load cell scale. It can be used with multiple digital load cells. It also includes a new junction box that contains four (4) A/D PCBs, each handling one analog load cell, which can connect through a standard POWERCELL type network or some other arrangement.
During calibration, the xe2x80x9cempty platformxe2x80x9d and xe2x80x9ctest loadxe2x80x9d readings for each load cell are stored in the instrument. The load cell symmetries, if any, existing in the system must also be stored in the instrument. For example, a vehicle or railroad track scale has left-right loading symmetry along the longitudinal axis; a tank scale could have radial symmetry; a floor scale or monorail scale usually has little or no symmetry.
A new Calibration Management Utility program is run at periodic intervals by the user or service personnel. This utility is primarily intended to help users meet ISO 9000 requirements for tractability of instrument calibration, with differences between actual and expected individual scale readings being flagged by the instrument and reported to the user as xe2x80x9cOKxe2x80x9d, xe2x80x9cMarginalxe2x80x9d, or xe2x80x9cBadxe2x80x9d. The Calibration Management Utility program can also be used to send messages when the scale needs to be audited manually, based on a decision event or criteria such as the number of weighing cycles or the passage of time.
As the weighing scale is operated, the instrument automatically makes periodic tests to determine the health of the load cells in the system. This includes examination of the load cell readings when the scale is xe2x80x9cemptyxe2x80x9d, as indicated by the operator request to zero the scale. It also includes a xe2x80x9csanity checkxe2x80x9d which looks at all of the load cells in the system and, knowing something about the symmetry of the scale and from past experience, it looks for any single load cell with a reading that is out of sync as compared to other load cell readings. Any error condition is analyzed.
A log file is employed to record xe2x80x9ceventsxe2x80x9d. An event is any detected failure, any suspicious reading, overload condition, or anything that has been defined to the system. The event log can be examined on command.
The weighing apparatus is network enabled. This means that there is connection to a customer LAN via Ethernet and to the Internet. There is also provision for a dial up modem connection to the Internet. The weighing apparatus can compose and send an email message to any designated party (e.g., a local service office) indicating that there is a problem or that the scale needs to be audited and indicate the urgency, either immediate or future. The network connection also permits remote access to the weighing apparatus, across a LAN or the Internet, which includes the ability to examine the event log, adjust settings, or just operate the instrument.
Analog load cell systems can fail catastrophically due to lightning, water ingress, or mechanical shock, but more often the failure is by degrees over a period of time. The most commonly used diagnostic of either current or impending failure is a shift in load cell output. This is done by periodically recording the load cell output when the scale is known to be empty and the output given a known test load, then comparing the readings to the values established when the scale was installed to make a determination about the health of the system.
Load cell output testing at no load condition (zero test) can be automated if the scale system periodically returns to a no load state. Hopper scales may confound the xe2x80x9cemptyxe2x80x9d condition test by accumulating material on the hopper surfaces. At odd intervals the material either falls away naturally, or an operator pounds the side of the hopper to force the issue. In cases such as storage tanks, the scale may never achieve a no load state.
Data can be obtained from individual load cells, so that each can be diagnosed and replaced if necessary without the need for disconnecting and reconnecting the entire scale or multi-load cell assembly. The diagnosis can be handled from a remote location by connection of a computer to the master controller through telephone lines and modems.
If the system consists of multiple load cells, each having a dedicated A/D channel, it may be possible to infer additional diagnostic information, depending on the known symmetries of the system. The following symmetry cases should be considered.
No symmetryxe2x80x94A floor scale or an overhead monorail scale are good examples. A load could be placed at any location. Any single cell could see all, some, or none of the load for any given weighment.
Left-right symmetryxe2x80x94A railroad track scale or (sometimes) a vehicle scale are good examples. The scale is made using two or more pairs of load cells. Each cell of a pair usually sees the same loading pattern.
Radial symmetryxe2x80x94Cylindrical tank or hopper scales often have identical net weight loading on all load cells, though they sometimes have an off center dead load due to the mounting of the discharge feeder machinery. This symmetry is especially useful if liquid materials are being weighed.
Multiple load cell scale systems having known symmetries can cross check the health of the load cells. Each cell could compute the likely reading of the other cells within the symmetry. If the readings do not match (within a tolerance), a fault condition is likely. If there are more than two cells in the symmetry, the most likely to be faulty cell can be selected by voting.
A maintenance log file must be separate from an event log for use by Weights and Measures. As example of some things to log or count in the maintenance log file are as follows:
1. Overloadxe2x80x94log all instances of a cell (or platform) being loaded above a preset static load limit.
2. Impactxe2x80x94log all occurrences of high impact loading (derivative exceeds preset limit AND overload exceeds present limit)
3. Weighment counter
4. Zero command counter
5. Zero command failure counterxe2x80x94used in conjunction with the Zero command counter.
The method for automated load cell failure prediction as specified above is more efficient, effective, accurate and functional than the current art.